Coking is one of the older refining processes. The purpose of a coke plant is to convert heavy residual oils (e.g. tar, asphalt, etc.) into lighter, more valuable motor fuel blending stocks. Refinery coking is controlled, severe, thermal cracking. It is a process in which the high molecular weight hydrocarbon residue (normally from the bottoms of the vacuum flasher in a refinery crude unit) are cracked or broken up into smaller and more valuable hydrocarbons.
Coking is accomplished by subjecting the feed charge to an extreme temperature of approximately 930° F. that initiates the cracking process. The light hydrocarbons formed as a result of the cracking process flash off and are separated in conventional fractionating equipment. The material that is left behind after cracking is coke, which is almost pure carbon. In addition to coke, which is of value in the metal industry in the manufacture of electrodes, fuel coke, titanium dioxide, etc., the products of a coke plant include gas (refinery fuel and LPG), unstabilized (wild) gasoline, light gas oil, and heavy gas oil.
The lion's share of the world's coking capacity is represented by delayed coking processes. Delayed coking can be thought of as a continuous batch reaction. The process makes use of paired coke drums. One drum (the active drum) is used as a reaction vessel for the thermal cracking of residual oils. This active drum slowly fills with coke as the cracking process proceeds. While the active drum is being filled with coke, a second drum (the inactive drum) is in the process of having coke removed from it. The coke drums are sized so that by the time the active drum is filled with coke, the inactive drum is empty. The process flow is then switched to the empty drum, which becomes the active drum. The full drum becomes the inactive drum and is emptied or decoked. By switching the process flow back and forth between the two drums in this way, the coking operation can continue uninterrupted.
After being heated in a direct-fired furnace, the oil is charged to the bottom of the active coke drum. The cracked light hydrocarbons rise to the top of the drum where they are removed and charged to a fractionator for separation. The heavier hydrocarbons are left behind, and the retained heat causes them to crack to coke.
A closed blowdown system is often used in delayed coker quench operations to support offline coke drum operations such as, for example, water-quenching operations and back-warming operations. In FIG. 1, a schematic diagram illustrates one example of a delayed coking quench system and a closed blowdown system.
The delayed coking quench system includes a pair of coke drums 102 and 104, a coke furnace 106 and a fractionator 108. Quench water 101a is introduced into coke drum 102, which is offline and ready for quenching. Although coke drum 102 is offline and coke drum 104 is online, each coke drum alternates between an online and an offline status depending on the status of the other coke drum. Therefore, if coke drum 104 is offline, then the quench water 101a would be introduced into coke drum 104. Effluent 106a from a furnace 106 is sent toward the coke drums 102 and 104. A switch valve 101b is used to direct the effluent 106a to the online coke drum, which is coke drum 104 in this example. A preheated hydrocarbon feed (not shown) enters the bottom of the fractionator 108, which provides surge time for the hydrocarbon feed before it is sent to the coke furnace 106. The coke furnace 106 typically heats the hydrocarbon feed up to about 930° F., which initiates the coking reactions in the coke furnace 106. This process forms the effluent 106a in the coke furnace 106, which is now a three-phase stream containing oil, undergoing reaction, vapor and some coke fines also referred to as hydrocarbon particulates. As the effluent 106a from the coke furnace 106 enters the online coke drum 104, solid coke begins to build in the coke drum 104 as effluent 106a flows through the channels in the coke bed building up in the coke drum 104. When the coke level in the coke drum 104 reaches a predetermined height in the coke drum 104, then the switch valve 101b is used to cut off effluent 106a from further entering the coke drum 104 and direct the effluent 106a to the recently emptied coke drum 102 that is offline. In this manner, coke drum 104 then becomes the offline coke drum and coke drum 102 becomes the online coke drum.
Hot vapors leaving the online coke drum 104 are quenched immediately upon leaving the coke drum 104 to kill the coking reactions, by a controlled injection of oil from the process. This forms the overhead hydrocarbon/steam stream 103 that is sent back to the fractionator 108 through isolation valve 105d in a switchdeck comprising isolation values 105a-105d. The fractionator 108 separates the quenched coke drum overhead stream 104a into heavy gas oil, light gas oil and overhead products using fractionation techniques well known in the art. The offline coke drum 102 is steam stripped and the overhead hydrocarbon/steam stream 103 is sent to the fractionator 108 for about forty-five minutes before isolation valve 105c is closed and isolation valve 105a is opened to redirect the overhead hydrocarbon/steam stream 103 to the quench tower 110 for about another forty-five minutes. At this point, the coke drum 102 can begin the quenching process as an offline coke drum.
As the quench water 101a is introduced into the offline coke drum 102, the quench water 101a is vaporized to produce the overhead hydrocarbon/steam stream 103, containing less hydrocarbon. The overhead hydrocarbon/steam stream 103 passes through isolation valve 105a in the switchdeck to enter the quench tower 110. The quench water 101a is initially forced into the offline coke drum 102 at a lower rate that is slowly increased as the coke bed therein is cooled. The quench water 101a eventually will fill the offline coke drum 102 to about five feet above the coke bed level, which may still produce some steam in the overhead hydrocarbon/steam stream 103.
In the quench tower 110, the overhead hydrocarbon/steam stream 103 is reduced to a temperature of about 370° F. to minimize temperature variations in the quench tower 110. A quench tower overhead steam stream 107 substantially comprising steam exits the quench tower 110 and enters a blowdown condenser 112.
The blowdown condenser 112 simply condenses the quench tower overhead stream 107 to form a blowdown condenser outlet stream 109 that enters a blowdown settling drum 114.
In the settling drum 114, the blowdown condenser outlet stream 109 is separated into a sour water stream 111, a light slop oil stream 113 and a hydrocarbon vapor stream 115. The hydrocarbon vapor stream 115 is sent back to the fractionator 108. The light slop oil stream 113 is also returned to the fractionator 108. The sour water stream 111 is sent to a sour water stripper, which removes sulfides from the sour water stream 111.
The quench tower 110, blowdown condenser 112 and settling drum 114 are collectively referred to as the closed blowdown system. The pressure in the offline coke drum 102 is generally the same as the pressure in the closed blowdown system. At this point, the offline coke drum 102 is isolated from the closed blowdown system and is vented to the atmosphere. An ejector or small compressor may be used in a line containing the hydrocarbon vapor stream 115 to reduce the pressure in the closed blowdown system and offline coke drum 102 to about 2 psig or less prior to venting the offline coke drum 102 as required by current environmental regulation guidelines. Despite venting the offline coke drum 102 to the atmosphere at 2 psig, a plume of steam is produced that may contain hydrocarbons, possibly hydrogen sulfide, and coke fines. Maintaining a pressure of 2 psig in the offline coke drum 102 prior to venting to the atmosphere is also an issue because the coke drum pressure can spike due to continuing heat evolution from the coke bed after isolation from the closed blowdown system. On some older units, which start to vent at around 15 psig, noise is also a significant issue.
Alternatively, the delayed coking quench system illustrated in FIG. 1 may be modified to include a coke drum quench overflow stream. Although existing overflow systems are somewhat varied and similar equipment is not necessarily used, they all benefit from the procedure of overflowing a coke drum at the end of the quench operation. For example, existing overflow systems do not require an ejector or compressor at the end of the closed blowdown system to reduce pressure in the system. This ejector is used to pull the pressure in the blowdown system and coke drum down at the end of the quench operation to around 2 psig before the coke drum is isolated from the blowdown system and vented to atmosphere. The overflow stream reduces the exposure of the offline coke drum to the atmosphere and eliminates significant vapor venting. Nevertheless, problems with existing overflow schemes can include odors and gas releases or fires, plugging exchangers and residual coke fines in piping that are flushed into other equipment when the coke drums are returned to the fill cycle because the overflow stream is not filtered before entering the closed blowdown system.
Because many existing overflow systems have American Petroleum Institute (“API”) separators or other equipment open to the atmosphere, there can be an emission of hydrocarbons and hydrogen sulfide, which is a serious problem. When the overflow stream is sent through an air cooler without being properly filtered, the air cooler can plug, which is also a problem in some existing overflow systems. In parts of the piping system used by existing overflow systems, coke fines are often left after the overflow operation, which are then flushed into the quench tower or fractionator when returning to the normal valving arrangement. Heavy oil or tar balls can occur in the coke bed, and if these are carried out of the coke drum by the quench water, the downstream equipment will not function well, and will require cleaning.